Conventional incinerating kilns operate at a slight negative pressure and employ relatively simple seals to control the leakage of outside air into the kiln. However, when a kiln is used to thermally process mixed hazardous and low level transuranic wastes the sealing requirements become much more stringent due to the need to prevent the spread of contaminated material generated by the kiln to the outside environment. As a result of this requirement, a special positive seal system is necessary. One possible method of achieving this level of containment involves formulating the seal assemblage in a manner which places as a minimum three independent seals in series with the outlet from a seal air system located between the primary and secondary seal and with an inlet located between the secondary and tertiary seal. This arrangement results in the outlet chamber, between the primary and the second secondary seals, being pressurized significantly above kiln pressure, such that any leakage past the primary seal would be into the kiln. In addition, any leakage past the secondary seal would move into the inlet chamber, located between the secondary and tertiary seals, where air is pulled into the seal air system and thus, still be retained inside the tertiary seal.
The air scavenged by the seal air system from the inlet chamber passes through a cooler and a HEPA filter and then returns to the inlet chamber using a positive displacement blower. This bypass loop is used to establish and maintain the specified pressure in the outlet chamber. Intentional leakage in through the tertiary seal balances the system providing make up air for that lost into the kiln. This triple seal arrangement prevents back flow from the kiln during normal negative pressure operation, during potential abnormal positive pressure transients and during shutdown conditions.
Normal rotary kiln operating pressure is approximately 1 mm Hg (0.5 in. water) negative with respect to the kiln room. The hypothetical positive pressure transient for a seal air system design is 517 mm Hg (10 psi) positive.
The rotary kiln has several features which affect the seal and therefore, must be taken into consideration. First, during its operation, the kiln rotates around an axis which is typically inclined 2 degrees from the horizontal, and as the kiln rotates, it can experience an axial displacement of up to one inch. Second, the waste material undergoing incineration requires about one hour to traverse the length of the kiln during which time the material is heated to a temperature of 1800 degrees F. In normal operation, the kiln temperature ranges from 300 to 400 degrees F along its length between the seals at either end of the kiln. Thermal expansion of the kiln shell increases its length by approximately 0.58 in. between the seals when going from the cold to hot operating condition. Also, while undergoing this temperature variation, the shell diameter increases 0.30 in. Thermal expansion of the apparatus used to rotate the kiln causes the entire kiln to expand upward in going from a cold to hot condition by 0.1 in. These conditions lead to a series of motion requirements affecting a kiln seal which can be summarized as follows:
a. Rotation of the kiln shell; PA0 b. Axial motion of the kiln shell of approximately 1.0 in.; PA0 c. Axial expansion of the kiln shell approximately 0.58 in. between feed and discharge end seals; PA0 d. Diametral expansion of the kiln shell due to thermal expansion approximately 0.30 in.; PA0 e. Vertical displacement of the kiln center of rotation of approximately 0.10 in. at both seals.
The previous seal design experienced a number of deficiencies when subjected to the seal motion requirements listed above. These deficiencies included: an inability to accommodate the vertical shift of the center of rotation of the kiln, excess air leaking into the kiln through the seals, and thermal stresses on a thermal sleeve which separates the room environment from the kiln gas. These deficiencies resulted in failure of the seal under operating conditions. Therefore, it is the object of this invention to provide a rotating kiln seal which will reliably operate under the above listed conditions while preventing leakage of contaminants generated by the incinerated waste.
A further object of this invention is to provide a seal which will operate under the inherent operating temperatures of the kiln and its exterior shell.
A further object of this invention is to provide a seal design employing a seal air system.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.